The present invention relates to a pressure-reducing device which may be used most satisfactorily in a case where, in relation to a hydraulic actuating system provided with more than one circuit module, a pressure lower than that set by a main relief valve is required for some of these circuit modules.
For example, in the case where an adjustment of the cylinder force is required for locking or clamping purposes and where it is impractical to adjust the main relief valve because of its relation to other circuit modules, the locking or clamping force was adjusted in the prior art by installing a pressure-reducing valve between the main circuit and the circuit concerned, so as to adjust its secondary pressure independently of the main circuit pressure.
In this case, the conventional pressure-reducing valve most commonly employed is of a construction that causes a spring or a pilot valve to be subjected to a pressure, and to a part of the flow rate related to the increase in the secondary pressure, so as to move a main spool which effects a reduction or a closing of the passage. In the case of such a pressure-reducing valve employed in the aforementioned locking or clamping circuit, therefore, the load pressure is normally low until just before the locking or clamping effect takes place and the spool, which is the pressure-reducing member of the pressure-reducing valve, is in the fully opened position in which it keeps the main circuit pressure at the same level as the load pressure. In this case, however, as soon as the locking or clamping effect takes place, a pressure increase takes effect which is detected by the pressure-reducing valve, which not only reduces its own effective cross-section in order to keep the secondary pressure at the set value, but also works to increase the main circuit pressure on the primary side so that it reaches the set pressure value of the main relief valve. Since the pressure-reducing valve generally starts to affect the main circuit flow from the secondary circuit after the load pressure reaches the set value, however, the pressure increase on the secondary side continues until the passage is fully closed. When it is fully closed, therefore, the pressure on the secondary side will become much higher than the set value, even though the passage may be closed very quickly.